Olympia Snowe to step down from US Senate
The senior US senator for the state of Maine, Olympia Snowe, will not seek re-election in November and is to retire after 33 years in Congress.
In a statement, Ms Snowe, seen as a moderate Republican, expressed frustration over an "atmosphere of polarisation" in Congress.
The move is seen as a boost for Democrats, who hope to retain their majority in the 100-seat Senate.
She said she had "no doubt" she would have won re-election.
Ms Snowe was first elected to the Senate in 1994, having previously served in the House of Representatives as well as in both bodies of the Maine state legislature.
Ms Snowe's retirement opens up the possibility of a Democrat winning her seat in November's election, analysts say.
Democrats currently control the Senate by a margin of 51 seats to 47, but have seven seats open for election in November - several seen as vulnerable to Republican challengers. With Ms Snowe's retirement, Republicans will have three open seats.
Fellow Maine senator and Republican Susan Collins called Ms Snowe's decision a "complete surprise".
In a statement, Ms Snowe said she saw the "vital need for the political center in order for our democracy to flourish and to find solutions that unite rather than divide us".
There were opportunities outside the Senate to serve her state, she said.
One Snowe campaign aide told Politico that Ms Snowe had struggled with her decision for weeks.
Candidates seeking her seat will declare by 15 March to meet a filing deadline.